Thursday, 7 August 2014

MA: Assembler directives

Assembly directives, also called pseudo-opcodes, pseudo-operations or pseudo-ops, are instructions that are executed by an assembler at assembly time, not by a CPU at run time. The names of pseudo-ops often start with a dot to distinguish them from machine instructions. Pseudo-ops can make the assembly of the program dependent on parameters input by a programmer, so that one program can be assembled different ways, perhaps for different applications. Or, a pseudo-op can be used to manipulate presentation of a program to make it easier to read and maintain. Another common use of pseudo-ops is to reserve storage areas for run-time data and optionally initialize their contents to known values.


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Assembler Directives

All assembler directives have names that begin with a period (`.'). The rest of the name is letters, usually in lower case.
This chapter discusses directives that are available regardless of the target machine configuration for the GNU assembler. Some machine configurations provide additional directives. See section Machine Dependent Features.

.abort

This directive stops the assembly immediately. It is for compatibility with other assemblers. The original idea was that the assembly language source would be piped into the assembler. If the sender of the source quit, it could use this directive tells as to quit also. One day .abort will not be supported.

.ABORT

When producing COFF output, as accepts this directive as a synonym for `.abort'.
When producing b.out output, as accepts this directive, but ignores it.

.align abs-expr, abs-expr, abs-expr

Pad the location counter (in the current subsection) to a particular storage boundary. The first expression (which must be absolute) is the alignment required, as described below.
The second expression (also absolute) gives the fill value to be stored in the padding bytes. It (and the comma) may be omitted. If it is omitted, the padding bytes are normally zero. However, on some systems, if the section is marked as containing code and the fill value is omitted, the space is filled with no-op instructions.
The third expression is also absolute, and is also optional. If it is present, it is the maximum number of bytes that should be skipped by this alignment directive. If doing the alignment would require skipping more bytes than the specified maximum, then the alignment is not done at all. You can omit the fill value (the second argument) entirely by simply using two commas after the required alignment; this can be useful if you want the alignment to be filled with no-op instructions when appropriate.
The way the required alignment is specified varies from system to system. For the a29k, hppa, m68k, m88k, w65, sparc, and Hitachi SH, and i386 using ELF format, the first expression is the alignment request in bytes. For example `.align 8' advances the location counter until it is a multiple of 8. If the location counter is already a multiple of 8, no change is needed.
For other systems, including the i386 using a.out format, it is the number of low-order zero bits the location counter must have after advancement. For example `.align 3' advances the location counter until it a multiple of 8. If the location counter is already a multiple of 8, no change is needed.
This inconsistency is due to the different behaviors of the various native assemblers for these systems which GAS must emulate. GAS also provides .balign and .p2align directives, described later, which have a consistent behavior across all architectures (but are specific to GAS).

.app-file string

.app-file (which may also be spelled `.file') tells as that we are about to start a new logical file. string is the new file name. In general, the filename is recognized whether or not it is surrounded by quotes `"'; but if you wish to specify an empty file name is permitted, you must give the quotes--"". This statement may go away in future: it is only recognized to be compatible with old as programs.

.ascii "string"...

.ascii expects zero or more string literals (see section Strings) separated by commas. It assembles each string (with no automatic trailing zero byte) into consecutive addresses.

.asciz "string"...

.asciz is just like .ascii, but each string is followed by a zero byte. The "z" in `.asciz' stands for "zero".

.balign[wl] abs-expr, abs-expr, abs-expr

Pad the location counter (in the current subsection) to a particular storage boundary. The first expression (which must be absolute) is the alignment request in bytes. For example `.balign 8' advances the location counter until it is a multiple of 8. If the location counter is already a multiple of 8, no change is needed.
The second expression (also absolute) gives the fill value to be stored in the padding bytes. It (and the comma) may be omitted. If it is omitted, the padding bytes are normally zero. However, on some systems, if the section is marked as containing code and the fill value is omitted, the space is filled with no-op instructions.
The third expression is also absolute, and is also optional. If it is present, it is the maximum number of bytes that should be skipped by this alignment directive. If doing the alignment would require skipping more bytes than the specified maximum, then the alignment is not done at all. You can omit the fill value (the second argument) entirely by simply using two commas after the required alignment; this can be useful if you want the alignment to be filled with no-op instructions when appropriate.
The .balignw and .balignl directives are variants of the .balign directive. The .balignw directive treats the fill pattern as a two byte word value. The .balignl directives treats the fill pattern as a four byte longword value. For example, .balignw 4,0x368d will align to a multiple of 4. If it skips two bytes, they will be filled in with the value 0x368d (the exact placement of the bytes depends upon the endianness of the processor). If it skips 1 or 3 bytes, the fill value is undefined.

.byte expressions

.byte expects zero or more expressions, separated by commas. Each expression is assembled into the next byte.

.comm symbol , length

.comm declares a common symbol named symbol. When linking, a common symbol in one object file may be merged with a defined or common symbol of the same name in another object file. If ld does not see a definition for the symbol--just one or more common symbols--then it will allocate length bytes of uninitialized memory. length must be an absolute expression. If ld sees multiple common symbols with the same name, and they do not all have the same size, it will allocate space using the largest size.
When using ELF, the .comm directive takes an optional third argument. This is the desired alignment of the symbol, specified as a byte boundary (for example, an alignment of 16 means that the least significant 4 bits of the address should be zero). The alignment must be an absolute expression, and it must be a power of two. If ld allocates uninitialized memory for the common symbol, it will use the alignment when placing the symbol. If no alignment is specified, as will set the alignment to the largest power of two less than or equal to the size of the symbol, up to a maximum of 16.
The syntax for .comm differs slightly on the HPPA. The syntax is `symbol .comm, length'; symbol is optional.

.data subsection

.data tells as to assemble the following statements onto the end of the data subsection numbered subsection (which is an absolute expression). If subsection is omitted, it defaults to zero.

.def name

Begin defining debugging information for a symbol name; the definition extends until the .endef directive is encountered.
This directive is only observed when as is configured for COFF format output; when producing b.out, `.def' is recognized, but ignored.

.desc symbol, abs-expression

This directive sets the descriptor of the symbol (see section Symbol Attributes) to the low 16 bits of an absolute expression.
The `.desc' directive is not available when as is configured for COFF output; it is only for a.out or b.out object format. For the sake of compatibility, as accepts it, but produces no output, when configured for COFF.

.dim

This directive is generated by compilers to include auxiliary debugging information in the symbol table. It is only permitted inside .def/.endef pairs.
`.dim' is only meaningful when generating COFF format output; when as is generating b.out, it accepts this directive but ignores it.

.double flonums

.double expects zero or more flonums, separated by commas. It assembles floating point numbers. The exact kind of floating point numbers emitted depends on how as is configured. See section Machine Dependent Features.

.eject

Force a page break at this point, when generating assembly listings.

.else

.else is part of the as support for conditional assembly; see section .if absolute expression. It marks the beginning of a section of code to be assembled if the condition for the preceding .if was false.

.endef

This directive flags the end of a symbol definition begun with .def.
`.endef' is only meaningful when generating COFF format output; if as is configured to generate b.out, it accepts this directive but ignores it.

.endif

.endif is part of the as support for conditional assembly; it marks the end of a block of code that is only assembled conditionally. See section .if absolute expression.

.equ symbol, expression

This directive sets the value of symbol to expression. It is synonymous with `.set'; see section .set symbol, expression.
The syntax for equ on the HPPA is `symbol .equ expression'.

.equiv symbol, expression

The .equiv directive is like .equ and .set, except that the assembler will signal an error if symbol is already defined.
Except for the contents of the error message, this is roughly equivalent to
.ifdef SYM
.err
.endif
.equ SYM,VAL

.err

If as assembles a .err directive, it will print an error message and, unless the -Z option was used, it will not generate an object file. This can be used to signal error an conditionally compiled code.

.extern

.extern is accepted in the source program--for compatibility with other assemblers--but it is ignored. as treats all undefined symbols as external.

.file string

.file (which may also be spelled `.app-file') tells as that we are about to start a new logical file. string is the new file name. In general, the filename is recognized whether or not it is surrounded by quotes `"'; but if you wish to specify an empty file name, you must give the quotes--"". This statement may go away in future: it is only recognized to be compatible with old as programs. In some configurations of as, .file has already been removed to avoid conflicts with other assemblers. See section Machine Dependent Features.

.fill repeat , size , value

result, size and value are absolute expressions. This emits repeat copies of size bytes. Repeat may be zero or more. Size may be zero or more, but if it is more than 8, then it is deemed to have the value 8, compatible with other people's assemblers. The contents of each repeat bytes is taken from an 8-byte number. The highest order 4 bytes are zero. The lowest order 4 bytes are value rendered in the byte-order of an integer on the computer as is assembling for. Each size bytes in a repetition is taken from the lowest order size bytes of this number. Again, this bizarre behavior is compatible with other people's assemblers.
size and value are optional. If the second comma and value are absent, value is assumed zero. If the first comma and following tokens are absent, size is assumed to be 1.

.float flonums

This directive assembles zero or more flonums, separated by commas. It has the same effect as .single. The exact kind of floating point numbers emitted depends on how as is configured. See section Machine Dependent Features.

.global symbol, .globl symbol

.global makes the symbol visible to ld. If you define symbol in your partial program, its value is made available to other partial programs that are linked with it. Otherwise, symbol takes its attributes from a symbol of the same name from another file linked into the same program.
Both spellings (`.globl' and `.global') are accepted, for compatibility with other assemblers.
On the HPPA, .global is not always enough to make it accessible to other partial programs. You may need the HPPA-only .EXPORT directive as well. See section HPPA Assembler Directives.

.hword expressions

This expects zero or more expressions, and emits a 16 bit number for each.
This directive is a synonym for `.short'; depending on the target architecture, it may also be a synonym for `.word'.

.ident

This directive is used by some assemblers to place tags in object files. as simply accepts the directive for source-file compatibility with such assemblers, but does not actually emit anything for it.

.if absolute expression

.if marks the beginning of a section of code which is only considered part of the source program being assembled if the argument (which must be an absolute expression) is non-zero. The end of the conditional section of code must be marked by .endif (see section .endif); optionally, you may include code for the alternative condition, flagged by .else (see section .else).
The following variants of .if are also supported:
.ifdef symbol
Assembles the following section of code if the specified symbol has been defined.
.ifndef symbol
.ifnotdef symbol
Assembles the following section of code if the specified symbol has not been defined. Both spelling variants are equivalent.

.include "file"

This directive provides a way to include supporting files at specified points in your source program. The code from file is assembled as if it followed the point of the .include; when the end of the included file is reached, assembly of the original file continues. You can control the search paths used with the `-I' command-line option (see section Command-Line Options). Quotation marks are required around file.

.int expressions

Expect zero or more expressions, of any section, separated by commas. For each expression, emit a number that, at run time, is the value of that expression. The byte order and bit size of the number depends on what kind of target the assembly is for.

.irp symbol,values...

Evaluate a sequence of statements assigning different values to symbol. The sequence of statements starts at the .irp directive, and is terminated by an .endr directive. For each value, symbol is set to value, and the sequence of statements is assembled. If no value is listed, the sequence of statements is assembled once, with symbol set to the null string. To refer to symbol within the sequence of statements, use \symbol.
For example, assembling
        .irp    param,1,2,3
        move    d\param,sp@-
        .endr
is equivalent to assembling
        move    d1,sp@-
        move    d2,sp@-
        move    d3,sp@-

.irpc symbol,values...

Evaluate a sequence of statements assigning different values to symbol. The sequence of statements starts at the .irpc directive, and is terminated by an .endr directive. For each character in value, symbol is set to the character, and the sequence of statements is assembled. If no value is listed, the sequence of statements is assembled once, with symbol set to the null string. To refer to symbol within the sequence of statements, use \symbol.
For example, assembling
        .irpc    param,123
        move    d\param,sp@-
        .endr
is equivalent to assembling
        move    d1,sp@-
        move    d2,sp@-
        move    d3,sp@-

.lcomm symbol , length

Reserve length (an absolute expression) bytes for a local common denoted by symbol. The section and value of symbol are those of the new local common. The addresses are allocated in the bss section, so that at run-time the bytes start off zeroed. Symbol is not declared global (see section .global symbol, .globl symbol), so is normally not visible to ld.
Some targets permit a third argument to be used with .lcomm. This argument specifies the desired alignment of the symbol in the bss section.
The syntax for .lcomm differs slightly on the HPPA. The syntax is `symbol .lcomm, length'; symbol is optional.

.lflags

as accepts this directive, for compatibility with other assemblers, but ignores it.

.line line-number

Change the logical line number. line-number must be an absolute expression. The next line has that logical line number. Therefore any other statements on the current line (after a statement separator character) are reported as on logical line number line-number - 1. One day as will no longer support this directive: it is recognized only for compatibility with existing assembler programs.
Warning: In the AMD29K configuration of as, this command is not available; use the synonym .ln in that context.
Even though this is a directive associated with the a.out or b.out object-code formats, as still recognizes it when producing COFF output, and treats `.line' as though it were the COFF `.ln' if it is found outside a .def/.endef pair.
Inside a .def, `.line' is, instead, one of the directives used by compilers to generate auxiliary symbol information for debugging.

.linkonce [type]

Mark the current section so that the linker only includes a single copy of it. This may be used to include the same section in several different object files, but ensure that the linker will only include it once in the final output file. The .linkonce pseudo-op must be used for each instance of the section. Duplicate sections are detected based on the section name, so it should be unique.
This directive is only supported by a few object file formats; as of this writing, the only object file format which supports it is the Portable Executable format used on Windows NT.
The type argument is optional. If specified, it must be one of the following strings. For example:
.linkonce same_size
Not all types may be supported on all object file formats.
discard
Silently discard duplicate sections. This is the default.
one_only
Warn if there are duplicate sections, but still keep only one copy.
same_size
Warn if any of the duplicates have different sizes.
same_contents
Warn if any of the duplicates do not have exactly the same contents.

.ln line-number

`.ln' is a synonym for `.line'.

.mri val

If val is non-zero, this tells as to enter MRI mode. If val is zero, this tells as to exit MRI mode. This change affects code assembled until the next .mri directive, or until the end of the file. See section Assemble in MRI Compatibility Mode: -M.

.list

Control (in conjunction with the .nolist directive) whether or not assembly listings are generated. These two directives maintain an internal counter (which is zero initially). .list increments the counter, and .nolist decrements it. Assembly listings are generated whenever the counter is greater than zero.
By default, listings are disabled. When you enable them (with the `-a' command line option; see section Command-Line Options), the initial value of the listing counter is one.

.long expressions

.long is the same as `.int', see section .int expressions.

.macro

The commands .macro and .endm allow you to define macros that generate assembly output. For example, this definition specifies a macro sum that puts a sequence of numbers into memory:
        .macro  sum from=0, to=5
        .long   \from
        .if     \to-\from
        sum     "(\from+1)",\to
        .endif
        .endm
With that definition, `SUM 0,5' is equivalent to this assembly input:
        .long   0
        .long   1
        .long   2
        .long   3
        .long   4
        .long   5
.macro macname
.macro macname macargs ...
Begin the definition of a macro called macname. If your macro definition requires arguments, specify their names after the macro name, separated by commas or spaces. You can supply a default value for any macro argument by following the name with `=deflt'. For example, these are all valid .macro statements:
.macro comm
Begin the definition of a macro called comm, which takes no arguments.
.macro plus1 p, p1
.macro plus1 p p1
Either statement begins the definition of a macro called plus1, which takes two arguments; within the macro definition, write `\p' or `\p1' to evaluate the arguments.
.macro reserve_str p1=0 p2
Begin the definition of a macro called reserve_str, with two arguments. The first argument has a default value, but not the second. After the definition is complete, you can call the macro either as `reserve_str a,b' (with `\p1' evaluating to a and `\p2' evaluating to b), or as `reserve_str ,b' (with `\p1' evaluating as the default, in this case `0', and `\p2' evaluating to b).
When you call a macro, you can specify the argument values either by position, or by keyword. For example, `sum 9,17' is equivalent to `sum to=17, from=9'.
.endm
Mark the end of a macro definition.
.exitm
Exit early from the current macro definition.
\@
as maintains a counter of how many macros it has executed in this pseudo-variable; you can copy that number to your output with `\@', but only within a macro definition.

.nolist

Control (in conjunction with the .list directive) whether or not assembly listings are generated. These two directives maintain an internal counter (which is zero initially). .list increments the counter, and .nolist decrements it. Assembly listings are generated whenever the counter is greater than zero.

.octa bignums

This directive expects zero or more bignums, separated by commas. For each bignum, it emits a 16-byte integer.
The term "octa" comes from contexts in which a "word" is two bytes; hence octa-word for 16 bytes.

.org new-lc , fill

Advance the location counter of the current section to new-lc. new-lc is either an absolute expression or an expression with the same section as the current subsection. That is, you can't use .org to cross sections: if new-lc has the wrong section, the .org directive is ignored. To be compatible with former assemblers, if the section of new-lc is absolute, as issues a warning, then pretends the section of new-lc is the same as the current subsection.
.org may only increase the location counter, or leave it unchanged; you cannot use .org to move the location counter backwards.
Because as tries to assemble programs in one pass, new-lc may not be undefined. If you really detest this restriction we eagerly await a chance to share your improved assembler.
Beware that the origin is relative to the start of the section, not to the start of the subsection. This is compatible with other people's assemblers.
When the location counter (of the current subsection) is advanced, the intervening bytes are filled with fill which should be an absolute expression. If the comma and fill are omitted, fill defaults to zero.

.p2align[wl] abs-expr, abs-expr, abs-expr

Pad the location counter (in the current subsection) to a particular storage boundary. The first expression (which must be absolute) is the number of low-order zero bits the location counter must have after advancement. For example `.p2align 3' advances the location counter until it a multiple of 8. If the location counter is already a multiple of 8, no change is needed.
The second expression (also absolute) gives the fill value to be stored in the padding bytes. It (and the comma) may be omitted. If it is omitted, the padding bytes are normally zero. However, on some systems, if the section is marked as containing code and the fill value is omitted, the space is filled with no-op instructions.
The third expression is also absolute, and is also optional. If it is present, it is the maximum number of bytes that should be skipped by this alignment directive. If doing the alignment would require skipping more bytes than the specified maximum, then the alignment is not done at all. You can omit the fill value (the second argument) entirely by simply using two commas after the required alignment; this can be useful if you want the alignment to be filled with no-op instructions when appropriate.
The .p2alignw and .p2alignl directives are variants of the .p2align directive. The .p2alignw directive treats the fill pattern as a two byte word value. The .p2alignl directives treats the fill pattern as a four byte longword value. For example, .p2alignw 2,0x368d will align to a multiple of 4. If it skips two bytes, they will be filled in with the value 0x368d (the exact placement of the bytes depends upon the endianness of the processor). If it skips 1 or 3 bytes, the fill value is undefined.

.psize lines , columns

Use this directive to declare the number of lines--and, optionally, the number of columns--to use for each page, when generating listings.
If you do not use .psize, listings use a default line-count of 60. You may omit the comma and columns specification; the default width is 200 columns.
as generates formfeeds whenever the specified number of lines is exceeded (or whenever you explicitly request one, using .eject).
If you specify lines as 0, no formfeeds are generated save those explicitly specified with .eject.

.quad bignums

.quad expects zero or more bignums, separated by commas. For each bignum, it emits an 8-byte integer. If the bignum won't fit in 8 bytes, it prints a warning message; and just takes the lowest order 8 bytes of the bignum.
The term "quad" comes from contexts in which a "word" is two bytes; hence quad-word for 8 bytes.

.rept count

Repeat the sequence of lines between the .rept directive and the next .endr directive count times.
For example, assembling
        .rept   3
        .long   0
        .endr
is equivalent to assembling
        .long   0
        .long   0
        .long   0

.sbttl "subheading"

Use subheading as the title (third line, immediately after the title line) when generating assembly listings.
This directive affects subsequent pages, as well as the current page if it appears within ten lines of the top of a page.

.scl class

Set the storage-class value for a symbol. This directive may only be used inside a .def/.endef pair. Storage class may flag whether a symbol is static or external, or it may record further symbolic debugging information.
The `.scl' directive is primarily associated with COFF output; when configured to generate b.out output format, as accepts this directive but ignores it.

.section name

Use the .section directive to assemble the following code into a section named name.
This directive is only supported for targets that actually support arbitrarily named sections; on a.out targets, for example, it is not accepted, even with a standard a.out section name.
For COFF targets, the .section directive is used in one of the following ways:
.section name[, "flags"]
.section name[, subsegment]
If the optional argument is quoted, it is taken as flags to use for the section. Each flag is a single character. The following flags are recognized:
b
bss section (uninitialized data)
n
section is not loaded
w
writable section
d
data section
r
read-only section
x
executable section
If no flags are specified, the default flags depend upon the section name. If the section name is not recognized, the default will be for the section to be loaded and writable.
If the optional argument to the .section directive is not quoted, it is taken as a subsegment number (see section Sub-Sections).
For ELF targets, the .section directive is used like this:
.section name[, "flags"[, @type]]
The optional flags argument is a quoted string which may contain any combintion of the following characters:
a
section is allocatable
w
section is writable
x
section is executable
The optional type argument may contain one of the following constants:
@progbits
section contains data
@nobits
section does not contain data (i.e., section only occupies space)
If no flags are specified, the default flags depend upon the section name. If the section name is not recognized, the default will be for the section to have none of the above flags: it will not be allocated in memory, nor writable, nor executable. The section will contain data.
For ELF targets, the assembler supports another type of .section directive for compatibility with the Solaris assembler:
.section "name"[, flags...]
Note that the section name is quoted. There may be a sequence of comma separated flags:
#alloc
section is allocatable
#write
section is writable
#execinstr
section is executable

.set symbol, expression

Set the value of symbol to expression. This changes symbol's value and type to conform to expression. If symbol was flagged as external, it remains flagged (see section Symbol Attributes).
You may .set a symbol many times in the same assembly.
If you .set a global symbol, the value stored in the object file is the last value stored into it.
The syntax for set on the HPPA is `symbol .set expression'.

.short expressions

.short is normally the same as `.word'. See section .word expressions.
In some configurations, however, .short and .word generate numbers of different lengths; see section Machine Dependent Features.

.single flonums

This directive assembles zero or more flonums, separated by commas. It has the same effect as .float. The exact kind of floating point numbers emitted depends on how as is configured. See section Machine Dependent Features.

.size

This directive is generated by compilers to include auxiliary debugging information in the symbol table. It is only permitted inside .def/.endef pairs.
`.size' is only meaningful when generating COFF format output; when as is generating b.out, it accepts this directive but ignores it.

.sleb128 expressions

sleb128 stands for "signed little endian base 128." This is a compact, variable length representation of numbers used by the DWARF symbolic debugging format. See section .uleb128 expressions.

.skip size , fill

This directive emits size bytes, each of value fill. Both size and fill are absolute expressions. If the comma and fill are omitted, fill is assumed to be zero. This is the same as `.space'.

.space size , fill

This directive emits size bytes, each of value fill. Both size and fill are absolute expressions. If the comma and fill are omitted, fill is assumed to be zero. This is the same as `.skip'.
Warning: .space has a completely different meaning for HPPA targets; use .block as a substitute. See HP9000 Series 800 Assembly Language Reference Manual (HP 92432-90001) for the meaning of the .space directive. See section HPPA Assembler Directives, for a summary.
On the AMD 29K, this directive is ignored; it is accepted for compatibility with other AMD 29K assemblers.
Warning: In most versions of the GNU assembler, the directive .space has the effect of .block See section Machine Dependent Features.

.stabd, .stabn, .stabs

There are three directives that begin `.stab'. All emit symbols (see section Symbols), for use by symbolic debuggers. The symbols are not entered in the as hash table: they cannot be referenced elsewhere in the source file. Up to five fields are required:
string
This is the symbol's name. It may contain any character except `\000', so is more general than ordinary symbol names. Some debuggers used to code arbitrarily complex structures into symbol names using this field.
type
An absolute expression. The symbol's type is set to the low 8 bits of this expression. Any bit pattern is permitted, but ld and debuggers choke on silly bit patterns.
other
An absolute expression. The symbol's "other" attribute is set to the low 8 bits of this expression.
desc
An absolute expression. The symbol's descriptor is set to the low 16 bits of this expression.
value
An absolute expression which becomes the symbol's value.
If a warning is detected while reading a .stabd, .stabn, or .stabs statement, the symbol has probably already been created; you get a half-formed symbol in your object file. This is compatible with earlier assemblers!
.stabd type , other , desc
The "name" of the symbol generated is not even an empty string. It is a null pointer, for compatibility. Older assemblers used a null pointer so they didn't waste space in object files with empty strings. The symbol's value is set to the location counter, relocatably. When your program is linked, the value of this symbol is the address of the location counter when the .stabd was assembled.
.stabn type , other , desc , value
The name of the symbol is set to the empty string "".
.stabs string , type , other , desc , value
All five fields are specified.

.string "str"


Copy the characters in str to the object file. You may specify more than one string to copy, separated by commas. Unless otherwise specified for a particular machine, the assembler marks the end of each string with a 0 byte. You can use any of the escape sequences described in section Strings.

.symver

Use the .symver directive to bind symbols to specific version nodes within a source file. This is only supported on ELF platforms, and is typically used when assembling files to be linked into a shared library. There are cases where it may make sense to use this in objects to be bound into an application itself so as to override a versioned symbol from a shared library.
For ELF targets, the .symver directive is used like this:
.symver name, name2@nodename
In this case, the symbol name must exist and be defined within the file being assembled. The .versym directive effectively creates a symbol alias with the name name2@nodename, and in fact the main reason that we just don't try and create a regular alias is that the @ character isn't permitted in symbol names. The name2 part of the name is the actual name of the symbol by which it will be externally referenced. The name name itself is merely a name of convenience that is used so that it is possible to have definitions for multiple versions of a function within a single source file, and so that the compiler can unambiguously know which version of a function is being mentioned. The nodename portion of the alias should be the name of a node specified in the version script supplied to the linker when building a shared library. If you are attempting to override a versioned symbol from a shared library, then nodename should correspond to the nodename of the symbol you are trying to override.

.tag structname

This directive is generated by compilers to include auxiliary debugging information in the symbol table. It is only permitted inside .def/.endef pairs. Tags are used to link structure definitions in the symbol table with instances of those structures.
`.tag' is only used when generating COFF format output; when as is generating b.out, it accepts this directive but ignores it.

.text subsection

Tells as to assemble the following statements onto the end of the text subsection numbered subsection, which is an absolute expression. If subsection is omitted, subsection number zero is used.

.title "heading"

Use heading as the title (second line, immediately after the source file name and pagenumber) when generating assembly listings.
This directive affects subsequent pages, as well as the current page if it appears within ten lines of the top of a page.

.type int

This directive, permitted only within .def/.endef pairs, records the integer int as the type attribute of a symbol table entry.
`.type' is associated only with COFF format output; when as is configured for b.out output, it accepts this directive but ignores it.

.val addr

This directive, permitted only within .def/.endef pairs, records the address addr as the value attribute of a symbol table entry.
`.val' is used only for COFF output; when as is configured for b.out, it accepts this directive but ignores it.

.uleb128 expressions

uleb128 stands for "unsigned little endian base 128." This is a compact, variable length representation of numbers used by the DWARF symbolic debugging format. See section .sleb128 expressions.

.word expressions

This directive expects zero or more expressions, of any section, separated by commas.
The size of the number emitted, and its byte order, depend on what target computer the assembly is for.

Warning: Special Treatment to support Compilers
Machines with a 32-bit address space, but that do less than 32-bit addressing, require the following special treatment. If the machine of interest to you does 32-bit addressing (or doesn't require it; see section Machine Dependent Features), you can ignore this issue.
In order to assemble compiler output into something that works, as occasionlly does strange things to `.word' directives. Directives of the form `.word sym1-sym2' are often emitted by compilers as part of jump tables. Therefore, when as assembles a directive of the form `.word sym1-sym2', and the difference between sym1 and sym2 does not fit in 16 bits, as creates a secondary jump table, immediately before the next label. This secondary jump table is preceded by a short-jump to the first byte after the secondary table. This short-jump prevents the flow of control from accidentally falling into the new table. Inside the table is a long-jump to sym2. The original `.word' contains sym1 minus the address of the long-jump to sym2.
If there were several occurrences of `.word sym1-sym2' before the secondary jump table, all of them are adjusted. If there was a `.word sym3-sym4', that also did not fit in sixteen bits, a long-jump to sym4 is included in the secondary jump table, and the .word directives are adjusted to contain sym3 minus the address of the long-jump to sym4; and so on, for as many entries in the original jump table as necessary.

Deprecated Directives

One day these directives won't work. They are included for compatibility with older assemblers.
.abort
.app-file
.line


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